Can't buy Crusader

Hunter1911:   The experience you have on the Cruiser is undistributed experience. So you have two options, you can spend that experience on unlocking the equipement on the vehicle itself, or unlocking the following vehicle. You can not skip vehicles, and unlock a vehicle several tiers higher unless you convert the undistributed experience via gold to Free experience, which you may use anywhere you deem fit.   So you either need collect the remaining experience needed on the Covenanter to unlock the Crusader. Or you may convert the experience from Cruiser to Fee experience by using gold.   But I would recommend the option 1. as you will get to know the vehicle branch more, and prepare better for the next tier.